1. Field of the Invention
The present invention relates to a high-performance gas filter assembly capable of maintaining a working atmosphere of high purity, which is required in the production of electronic devices, thereby contributing to the enhancement of the product quality, and is also capable of checking dust generation from the filter paper used in this environment. 2. Description of the Prior Art
In the electronic, precision, and pharmaceutical industries, the manufacture of products in which adhesion of dust exerts a serious influence on the product, requires a dust-free atmosphere of high degree. To meet this demand, filter paper having a high performance such that particles of 0.3 .mu. in diameter can be tapped at an efficiency of more than 99.97% has been developed.
However, in manufacture of paper from fine glass fibers or synthetic fibers, which is a technique generally used in the manufacture of high-performance filter paper, the purity of the air is not as high as expected from the performance of the filter paper considered to have caught almost all of the dust. Also, residue such as fibers from the manufacture of the filter paper contaminate the filter paper and thus generate dust when a blast of air or vibration is applied to the filter creating dust downstream of the filter. For this reason, it appears that there is a substantial reduction in trapping efficiency using glass and synthetic fiber filters by themselves.
Filter paper generally has a large thickness (e.g., 400 .mu.) in order to increase the filtering capacity. From the time in which the filter paper is used it is believed that almost all dust which has flowed into the filter is trapped between the air entrance surface and an area about 200 .mu. deep from the entrance surface, as indicated by the dark area in the sectional view of FIG. 1. The air exits through the filter paper, but in actuality, the air has not been purified to the expected purity.
Conventional filter paper is produced by mixing fibers such as glass fibers and synthetic fibers, which have diameters selected in accordance with the required performance, into water with an adhesive resin mixed therein, dehydrating the mixture and drying it. The water and adhesive resin mixture provide strength for the filter paper. With this method, it is impossible to prevent minute waste of fibers from being produced and mixed into the water during the course of production of the fibers. In addition, since a multiplicity of fine particles are contained in the water which is used for paper making, there are multiplicity of fine particles contained on the surface and in the interior of the paper filter which is produced by dehydrating and drying the paper.
It is possible to fix the fine particles contained on the surface and in the interior of the paper filter by means of the adhering action of the adhesive resin which is mixed into the water for the purpose of providing strength to the filter paper. When the amount of adhesive resin is increased so that the adhesive strength and mechanical strength of the bond between fibers is enhanced, the ratio of closing the pores produced between the fibers increases and the pressure drop across the filter also increases. Therefore, the amount of adhesive resin is generally limited to as small a value as about 7% with respect to the weight of filter paper. This value however is insufficient for fixing the total amount of the fine particles to the fiber surface, so that many fine particles are loose in the filter paper. Accordingly, when vibration or shock is applied to the filter paper due to blast or a pulse, the loose fine particles slip off the filter paper, thereby substantially lowering the purity of the air.
TABLE 1 ______________________________________ Immersion number of times 1 2 3 Particle diameter Counts ______________________________________ 0.1 to 0.5 68 100 111 0.5 to 0.75 648 741 763 0.75 to 1.0 67 97 83 1.0 to 2.0 71 74 69 2.0 to 5.0 38 32 44 5.0 to 10.0 28 28 27 6.0 to 10 or more 6 5 5 Total 926 1077 1102 ______________________________________
Table 1 shows the results of simple experiments for obtaining the content of fine particles in the filter paper. Standard high-performance filters, namely, filter paper made from glass fibers having an average diameter of about 0.4 to 0.7 .mu.m were tested for particles, a particle having a diameter of 0.3 .mu.m and a thickness of 0.4 mm, and a quoted trapping efficiency of 99.97%. The test method consisted of cutting the filter paper into sizes of 4 cm.times.4 cm, and immersing them in 100 cc of pure water three times, for 1 minute immersion and the fine particles found in the pure water were classified by diameter and the number of fine particles were measured (counted) for each classified particle diameter.
As is clear from Table 1, the fine particles contained in the filter paper were not removed by a one-time immersion. There was a tendency for the number of fine particles released from the filter paper to increase with the number of times of immersion, namely, the first immersion totalled 926; the second immersion totalled 1,077, and the third totalled 1,102. For each immersion, the fine particles having a particle diameter of 0.5 to 0.75 .mu. were removed in the maximum number. The total number of fine particles released from the filter paper by the three immersions was 3,105. When this is calculated in terms of a standard filter within a frame with the dimensions 610 mm (length) .times.610 mm (width) .times.150 mm (depth) having a total surface area (20 m.sup.2) trapping efficiency of 99.97%, the total number of fine particles contained in the filter paper calculated is 3,881.times.10.sup.7. In addition, since not all of the fine particles in the filter paper had escaped even after the three-time immersion, the total number of fine particles contained in the filter paper may be greater than 3,881.times.10.sup.7. Since a small number of these particles are released due to blast or vibration, the overall performance of the filter paper is lowered.
As can be seen from the description above, the object of the invention is to produce a filter without fine particles contained in filter paper in order to improve the performance of conventional filter paper. This is difficult to accomplish. It is more practical to allow fine particles to be contained in filter paper and to prevent them from being released from the filter paper. It is also necessary to prevent the increase in the pressure drop and the lowering of the performance of filter paper due to the presence of the fine particles in the filter paper.
Porous films of synthetic resins with pores having the necessary diameter, and in which the porosity is at a maximum of about 30% are available presently. However, while these porous films may be placed on high-performance filter paper so that they prevent the release of the fine particles contained in the filter paper, they also increase the pressure drop so much they are not suitable to be used with the conventional filter paper.